Sam Allardyce's legal representatives have sent letters to Steve Kean and Blackburn Rovers following video footage that shows the club's manager making disparaging remarks about his former colleague.

Kean was secretly videoed by supporters during Blackburn's trip to Hong Kong last summer. He tells those fans that Rovers will definitely finish in the top 10 (for the season that has just finished, when they were relegated), also win the Carling Cup and places a £100 bet with them. However, it is the potentially libellous comments about Allardyce, who was in charge when Kean was a coach at Ewood Park, that have seen legal proceedings begin.

Allardyce, now manager at West Ham United and preparing for Saturday's Championship play-off final with Blackpool, only became aware of the YouTube video yesterday morning. He met his legal team just after lunchtime and the decision was made to begin proceedings. The footage that has sparked the controversy was filmed months ago but its posting on Twitter has pushed it into the public domain. It shows Kean questioning the operating methods and ability of the man who was manager before him at Blackburn.

Kean and Allardyce fell out hugely after the latter was sacked by Venky's, Blackburn's owners, in December 2010, despite the team being 13th in the Premier League at the time. To the surprise of many, especially in light of promises of big-money signings such as Ronaldinho and speculation linking Diego Maradona with the vacant manager's position, the Indian owners instead promoted from within.

Kean, however, has faced a huge public backlash in Blackburn since he was given the job, with repeated demonstrations calling for him to be sacked or to resign. The local newspaper has twice called for him to go and his appointment has been blamed for the huge disappointment of the current campaign, which ended in relegation before the final game of the season.

The 44-year-old Scot was flying to Pune yesterday to meet Venky's owner Anuradha Desai for further talks about his position. It has been alleged that the relationship between Desai and Kean's wife Margaret was the reason he has kept his position at Ewood Park.

Kean insisted last week that he retained the support of the Rao family, which owns Venky's, despite Blackburn's demotion to the Championship, and that the controversial family were the right people to lead the club back into the Premier League.

A legal letter has also been sent to them from Allardyce's representatives as the employers of Kean. Allardyce is said to be furious with the slur. The release of the four-minute YouTube rant by Kean was calculated to cause maximum embarrassment as he fights for his job; the BRFC Action Group, who have repeatedly called for his dismissal and the removal of Venky's, posted the video on their website yesterday. They have also been granted meetings with the Government and the Premier League to air their concerns.